The present invention relates to a cathode-ray tube, and more particularly, an electrode structure of a cathode-ray tube utilizing an electric field for focusing of an electron beam.
An all-electrostatic camera tube (hereinafter referred toaas SS camera tube) uses an electric field for focus and deflection of an electron beam and is described in, for example, Kakizaki et al, "An All-Electro Static Camera Tube", Technical & Research Report of The Institute of Television Engineers of Japan, No. ED-808, Sept. 28, 1984, pp. 7-pp. 12, JP-A-No. 60-47351 and JP-A-No. 60-49542. The SS camera tube has merits that good characteristics can be achieved with a short tube length, that no coil assembly is required for focus and deflection, and that a power consumption required for focus and deflection of an electron beam is very small. Therefore, the SS camera tube is advantageous to the reduction in size, weight and power consumption of a video camera.
FIG. 2 shows a schematic cross-sectional view of the conventional SS camera tube. An electron beam emitted from an electron gun which is constituted by a triode section including a cathode 201, a first grid 202 and a second grid 203, is focused onto a photoconductive target 208 by the action of an electric field produced by third, fourth and fifth grid electrodes 205, 206 and 207 which are disposed on the inner wall of a glass tube 204. At the same time, the electron beam is deflected by the action of an electric field formed by deflection electrodes as the fourth grid electrodes 206 to scan the photoconductive target 208, thereby reading an image signal. The signal thus obtained is taken out of the glass tube 204 to the outside thereof through a pin 210 which passes through a glass plate 209. A mesh electrode 211 and a ring electrode 212 are connected to the same potential E.sub.C6 (V). An electrostatic lens is formed by a potential difference established between the ring electrode 212 and the fifth grid electrode 207 which is applied with a potential E.sub.C5 (V). This lens is called a collimating lens and has a function of adjusting a landing error of the deflected electron beam in a radial direction. The fifth grid electrode 207 and the mesh electrode 211 are applied with their voltages from the outside of the glass tube 204 via a metal pin 213 passing through the glass tube 204 and via an indium ring 214, respectively. Voltages to the other electrodes are supplied via stem pins 215.
FIG. 3 shows an expanded view of pattern electrodes (including the third to fifth grid electrodes) disposed on the inner wall of the tube. The third grid elecrrode 205 is constituted by leads H.sub.L.sup.+, H.sub.L.sup.-, V.sub.L.sup.+ and V.sub.L.sup.- of the deflection electrodes (fourth grid electrode) 206 and an interleaved electrode G.sub.Q applied with a potential E.sub.C3 (V). The fourth grid electrode 206 is constituted by horizontal-deflection electrodes H.sup.+ and H.sup.- and vertical-deflection electrodes V.sup.+ and V.sup.-. To the deflection electrodes H.sup.+, H.sup.-, V.sup.+ and V.sup.-, bias voltages E.sub.C4 superimposed with +V.sub.H /2, -V.sub.H /2, +V.sub.V /2 and -V.sub.V /2 are applied, respectively. The applied voltages form a deflection electric field. Typical voltages applied to the electrodes are as follows. The voltages of the second grid 203, interleaved electrode G.sub.Q, fifth grid electrode 207 and mesh electrode 211 are 105V, 800V, 480V and 800V, respectively. The electron beam is focused onto the target 208 by adjusting the bias voltage E.sub.C4.
When SS camera tubes are used for high-definition TV cameras for the purpose of providing a high resolution, the resolution in the case of using the conventional SS camera has a limitation due to the aberration of an octupole lens involved, as will be explained in below.
Explanation will now be made of the spherical aberration of an axial-symmetric lens and the aberration of the octupole lens.
FIG. 4 illustrates a distribution of potentials on a circumference A-A' shown in FIG. 3 when no deflection is made. The abscissa of FIG. 4 represents an angle .theta. around the tube axis and the ordinate thereof represents a potential .phi.. The potential .phi. can be regarded as one in which a potential changing alternately in positive and negative directions is superimposed on a potential V.sub.0 which is an average of the potential .phi. in the direction of .theta.. An axial-symmetric lens is formed by the potentials V.sub.O, E.sub.C4 and E.sub.C5 and an electron beam is focused onto the target by this lens. One of factors obstructing the focusing of the electron beam when only the axial-symmetric lens exists, is the spherical aberration of the axial-symmetric lens.
The spherical aberration of the axial-symmetric lens is schematically illustrated in FIGS. 5A and 5B. The trajectory 501 of the electron beam emitted with a small divergent angle from a central portion c of the second grid 203 is focused onto a paraxial image plane 520. However, as the divergent angle increases, a point of intersection of the beam trajectory with the tube axis moves toward the second grid 203, as is seen from trajectories 502 and 503 in FIG. 5A. Thus, as is shown in FIG. 5B, the impinging position of electrons on the paraxial image plane concentrically expands as the divergent angle of the electron beam is increased. Such an electron beam has the minimum diameter at a location 521 in front of the paraxial image plane. A circle having the minimum diameter is cllled a disk of least confusion. The above-mentioned phenomenon is called the aberration, especially, spherical aberration of the axial-symmetric lens. The divergent angle of an electron beam in the camera tube is several degrees at largest. With such divergent angles, a third-order spherical aberration is dominant. When only the third-order spherical aberration exits, the divergent angle .alpha. of electrons and the radius r of the impinging position of electrons on the paraxial image plane has the following relation: EQU r=MC.sub.s .alpha..sub.3 ( 1)
Here, M is the magnification of image formation of the lens and C.sub.s is the coefficient of the thrrd-order spherical aberration.
An electrostatic lens in the SS camera tube can be regarded a combined lens of an axial-symmetric lens and an octupole lens. The octupole lens is formed by an electric potential changing alternately in positive and negative directions above and below the average potential V.sub.O.
FIG. 6 represents an electric field formed by the octupole lens by vectors (or arrows) in a cross section taken along line A--A' of FIG. 3. It is seen from FIG. 6 that the electric field is directed from the interleaved electrode G.sub.Q toward the leads H.sub.L.sup.+, H.sub.L.sup.-, V.sub.L.sup.+ and V.sub.L.sup.- of the deflection electrodes so that the octupole lens is formed. It is also seen that the influence of the octupole lens is little in the vicinity of the tube axis but becomes large with approach to the tube wall. Accordingly, the addition of the octupole lens to the axial-symmetric lens results in no change of the paraxial characteristics (including the position of the paraxial image plane and the magnification of the lens) but influences the third-order aberration and gives a .theta.-direction dependency to the aberration. When electrons are emitted with a divergent angle .alpha. from the central portion c of the second grid 203 in the circumferential direction .theta., i.e., when the projections .beta. and .gamma. of the divergent angle of electrons to the x- and y-directions are give by the equations of EQU .beta.=.alpha. cos .theta. (2)
and EQU .gamma.=.alpha. sin .theta. (3)
the coordinates x.sub.b and y.sub.b of the impinging position of electrons on the paraxial image plane are as follows: EQU x.sub.b =MC.sub.sx (.theta.) .alpha..sup.3 ( 4) EQU y.sub.b =MC.sub.sy (.theta.) .alpha..sup.3 ( 5)
Here, M is the magnification and C.sub.sx (.theta.) and C.sub.sy (.theta.) are given by the following equations: EQU C.sub.sx (.theta.)=a cos.sup.3 .theta.+b cos.sup.2 .theta. sin .theta.+c cos .theta. sin.sup.2 .theta.+d sin.sup.3 .theta. (6) EQU C.sub.sy (.theta.)=-d cos.sup.3 .theta.+c cos.sup.2 .theta. sin .theta.-b cos .theta. sin.sup.2 .theta.+a sin.sup.3 .theta. (7)
In the case of an ordinary axial-symmetric lens, the equations of EQU a=c=C.sub.s ( 8)
and EQU b=d=O (9)
are satisfied. Therefore, deviations from the condition equations (8) and (9) can be regarded as influences of the octupole lens.
The present inventors have analyzed the influences by the octupole lens for the conventional SS camera tube shown in FIGS. 2 and 3. The result of the analysis gives the coefficients a, b, c and d of the third-order aberration which are 1.15 (.mu.m/deg.sup.3), 0, -0.75 and 0, respectively. This result significantly deviates from the equations (8) and (9) conditioning the axial-symmetric lens and therefore demonstrates great influence of the octupole lens. FIGS. 7A and 7B show the shapes of beam spots formed on the paraxial image planes. FIG. 7A illustrates the beam spot in the case where an effect of the octupole lens is involved while FIG. 7B illustrates the beam spot in the case where only the axial-symmetric lens exists. In both cases, the divergent angle of an electron beam from the electron gun was 2.2.degree.. From FIGS. 7A and 7B, it is seen that though the shape of the beam is circular when only the axial-symmetric lens exists, the otcupole lens has an effect of expanding the beam like four leaves.
FIGS. 8A and 8B show the shapes of electron beams when a circumscribed circle of the beam becomes to be minimum. In a usual operation, the bias voltage E.sub.C4 is adjusted so that the minimum circumscribed circle is positioned on the target surface. Therefore, the beam shapes shown in FIGS. 8A and 8B can be regarded as ones when the focusing of the electron beam is optimized. FIG. 8A illustrates the beam shape in the case where the effect of the octupole lens is put into consideration while FIG. 8B illustrates the beam shape in the case where only the axial-symmetric lens exists. Though the beam diameter is 1.8 .mu.m when only the axial-symmetric lens exists, the beam diameter under the presence of the octupole lens is increased to 5.1 .mu.m which is about three times of 1.8 .mu.m. Thus, the conventional SS camera tube has a disadvantage drawback that under the influence of the octupole lens formed by the interleaved electrode G.sub.Q and the lead portions H.sub.L.sup.+, H.sub.L.sup.-, V.sub.L.sup.+ and V.sub.L.sup.- of the deflection electrodes, the diameter of the electron beam is increased, thereby deteriorating the resolution. Further, when the defocusing of an electron beam takes place, the beam shape expands like four leaves, thereby resulting in the deterioration of characteristics that the resolution has a dependence on direction. The direction dependency of the resolution means that the resolution of the camera tube, for example, when the image of a black and white pattern consisting of oblique stripes is formed, the resolution changes depending on the tilt angle of the pattern.
As mentioned above, the conventional SS camera ttube has the drawbacks that due to the influence of aberration of the octupole lens, the diameter of an electron beam is increased resulting in the deterioration of resolution and that when the beam defocusing takes place, the dependence on direction is produced in the resolution.